TMZ has obtained a demand letter Horowitz sent to the New York Post, which ran a teaser for an upcoming article about Rachel and her deceased fiance, James Andrew O'Grady, who died during the 9/11 attack.

The teaser includes alleged statements from Rachel -- "I believe Andy was meant to die because he was too good," adding, "It would have been tragic if we got into fights and then divorced ... I'm glad I didn't get to see any flaws that time brings on everyone. I would otherwise be a fat housewife with three kids living in Sands Port, Long Island."

In his letter, Horowitz claims the story conveys "a disastrously false impression of my client," adding the story is "attacking, degrading and extraordinarily inaccurate."

Rachel tells TMZ she feels she's the victim of a hit piece and the reporter went into it not liking her.

Horowitz also has words for the author: "You created a work of miserable and demeaning fiction. You should be ashamed of yourself."

Horowitz wants an apology to Rachel, and wants the "false words" withdrawn, although he is not specific as to how they would be withdrawn. The full story is supposed to come out tomorrow.

We called the New York Post yesterday, and a rep said "We stand by our story."